View Full Version : Another woodworking machine query

Alan Smith
12-28-2006, 02:31 AM
Gentlemen you were very helpfull previously with a query regarding a disc sander I intended to build. That machine is still on the drawing board whilst I struggle with the current project. At the moment I am building a hollow chisel mortiser, or at least the power-head part. Essentially this is a 1HP motor with a Jacobs chuck on the spindle, into the chuck goes the rotating bit and concentric with this is the square hollow bit which chops out the corners of the mortise. The square bit is firmly held by a bracket clamped to the motor everything naturally lined up to be axial/concentric. My problem is that I have had to turn up an adaptor/collar to link the motor spindle to the Jacobs chuck. Either I have introduced an error with this collar or the motor spindle is slightly bent but the tip of the bit, which is about 12" from the motor is sweeping a circle of about 0.25". Using a Dial guage I think I can track this deviation back to the motor spindle. One solution other than finding a better motor is to convert the adapter collar to a flexible coupling. I wondered about parting the collar in half and connecting with some rubber hose and a couple of radiator hose clamps. The coupling has to be able to withstand axial pull as the rotating bit has a fierce upcutting spiral to clear the woodchips from the tool. Am I expecting too much of my simple flexible coupler idea or is there a better coupling solution or should I just search for a better motor, remember this is all being done on the cheap! Thanks in advance for any advice and a Happy New Year to all (That's Happy Hogmany to Alistair!)


12-28-2006, 05:16 AM
Could the bearings/bushings in your motor, be worn out? On most of the mortise machines that I have used or seen the motor's arbor has been turned to directly mount the chuck, (ie a JT#). If I understand your solution correctly, I think that you will have problems with "squiwsh" and "stalling" when cutting and then "staying together" when you attempt to retract the chisel from the mortise. Since the chisel is fixed, any "wobble" in the auger will cause it to rub against the inside of the chisel generating heat.

Alan Smith
12-28-2006, 06:35 AM
Techshop, motor bearings could be worn, it's a pretty old motor from a radial saw set up. My initial thoughts were to get the spindle/armiture out the motor and turn a fitting directly on the spindle end but I fear that my lathe may not have the capacity to do this but it still can be explored.

Your thoughts on squish and coming apart of the coupling were what I am concerned about which is why I was wondering about a more robust coupling.

Ian B
12-28-2006, 06:42 AM

The drill describing a 1/4" circle - it only does this when you don't have the square hollow chisel clamped in place, I assume?

In that case, could it be a bent drillbit? I have some of these mortice chisels - top Chinese quality. All of the drills are a bit off, but they run ok in the square chisels. A little oil on them helps.

Replace the drill with a piece of bar of known straightness, and try again. If it runs true, it's the drill.

If the chuck really is off, then it'll show a wobble on the dial gauge when you clock it (the chuck).

How did you make the motor spindle to chuck adaptor? Do you have a metal turning lathe?

How does the adaptor affix to the motor spindle; 1 or 2 grub screws? If the bore is a little oversize, this would easily account for the wobble, which a long drill will magnify. The solution would then be to make a tighter fitting adaptor. If you can, bore the hole with a small boring tool rather than drilling it; closer tolerance, more likely to be axial.



Your Old Dog
12-28-2006, 08:36 AM

In that case, could it be a bent drillbit? I have some of these mortice chisels - top Chinese quality. All of the drills are a bit off, but they run ok in the square chisels. A little oil on them helps.

Thats the way I do it.

Alan Smith
12-28-2006, 08:41 AM
Ian, the bit is a good quality Japanese bit. I don't think it's bent especially as I can track the wobble with the clock guage pretty well up to the motor spindle. You are right, squre chisel not clamped as I have not made the clamp/mount yet. The collar/adaptor was made on a small (very old) model makers lathe and is basically 1.25" dia round bar about 2.75" with a 15mm bore one side for the motor spindle and a 11mm bore t'other side for the Jacobs chuck spindle. Grub screws to locate on flats/keyways. The frustrating thing is that each individual component appears to be in truth, it's only when assembled that I get this run-out. I've messed around with the various grub screws but the fits are surprisingly good (for me) and I can't measure any variation by changing the screw tightness etc.

I really like the idea of machining the motor spindle, if for no other reason than it will reduce the overall length of the assembly and reduce the amplification of any intrinsic runout. I'll have a look at dismantling the motor this afternoon.

12-28-2006, 08:51 AM
Unless you machine your adapter, once fitted, as a unit with the motor shaft you are inviting errors of the kind you describe,

john hobdeclipe
12-28-2006, 09:06 AM
You need a rigid mounting to support the bit all the way through to a thrust bearing.The bit is NOT self feeding, so you'll need a substantial thrust bearing in the system to hold the bit away from the chisel. When in use, the bit needs to maintain a few thousandths (.005"-.010") clearance from the end of the chisel; they should never touch. The chisel is not intended to guide the drill.

Forget the drill chuck. Production mortisers always use a simple straight bore adapter with setscrew to hold the bit. Adding a chuck into the system just introduces another possibility for errors.

Conceivably, you could machine a flat end onto the end of the motor (around the shaft) then capture a thrust bearing between that and the drill adapter.

Alan Smith
12-28-2006, 12:15 PM
John what you say makes a lot of sense. I have to say that all the pictures I have looked at of mortisers appear to show that there is a Jacobs chuck fitted. I also assumed that different size bits would have differing shank diameters. Getting rid of the chuck in my set up would be helpful if possibly less flexible in the long run regarding different bit sizes. Your comments about thrust bearings also are sensible and I had already pencilled this in as a possibility mounted behind my adaptor and bearing on the plate that will connect the motor to the assembly that clamps the square chisel.

I managed to dismantle the motor this afternoon and can mount the spindle between centres on the lathe so in theory i can now machine the motor spindle directly. The problem with this is that the chuck that I want /(wanted) to use is Japanese and rather than a taper is tapped and has a threaded spindle. I can't identify the thread. Being Japanese (Hitachi) I assume it's metric, dia to crest of threads is 12.20mm/0.4820" and pitch is roughly 20tpi/0.8mm. I don't have the change wheels for my lathe so cutting threads is not possible anyway.

Looks like I might be going with John's idea of ditching the Jacobs chuck.

12-28-2006, 12:50 PM
i have a bench-mounted Delta mortiser. it has a chuck in it to hold the drill bit, and the chisel portion is held with set screws in a collar. while i have never ran the drill bit by itself, only while contained in the chisel, it does appear to exhibit that fine chinese wobble and makes a heck of a racket when running (some oil does quiet it down). i'd say try to make a mortise with your current set up and see how it goes. if the bit gets too hot or really starts squealing, then stop and attempt a different approach.

you could aways build a chain mortiser. :)

andy b.

Alistair Hosie
12-28-2006, 01:38 PM
Alan great Gentleman and a renowned scholar like your self knows Hogmany is new years eve not new year :D Have a nice one and buy a chisel morticer to strip down for parts:D it'll be cheaper min the long run:D have a great on your pal Big Al

Alan Smith
12-28-2006, 02:31 PM
Alistair, I'm an Aberdonian, the thought of having to open my wallet to buy anything more for this machine makes me shudder!!

Alan Smith
12-29-2006, 12:36 PM
Ok here's what I did. I adopted John Hobdeclipe's excellent idea and have done away with the Jacobs chuck. With the motor spindle held between centres on the lathe I built a wooden steady around the spindle bearing nearest the tailstock and made sure the motor spindle was running true. I then backed off the tailstock and mounted the adaptor. My lathe bed is not long enough to bore the adaptor so I skimmed it down to ensure that the outer surface was truely concentric with the motor spindle. Took the whole thing down and mounted the adapter only in the lathe chuck and bored the 0.5" hole for the mortiser bit. Assembled the whole thing and result is vast improvement, there is a detectable runout at the tip of the bit, about 1/32" but bearing in mind the bit is 10-11" long I reckon this is as good as I need. Now all I have to do is build the rest of the machine. Thanks for all the imput on this.


Alan Smith
01-25-2007, 04:40 AM
Well I thought you all might like to know that I cut my first mortise on this machine this morning. The work on it has been a bit stop start due to other work commitments but it has all come together and seems to work very well. There are a few details needing tweaked but all I really need to do now is find a source of green paint!!

Thank you all for you input on this, if I can sort out a camera I'll post a few pictures.

Thanks, Alan.